Highly luminescent color-selective nanocrystalline materials

ABSTRACT

A nanocrystal capable of light emission includes a nanoparticle having photoluminescence having quantum yields of greater than 30%.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.12/725,438, now U.S. Pat. No. 8,101,234, filed Mar. 16, 2010, which is acontinuation of U.S. patent application Ser. No. 12/509,869, now U.S.Pat. No. 8,158,193, filed Jul. 27, 2009, which is a continuation of U.S.patent application Ser. No. 11/502,493, filed on Aug. 11, 2006, now U.S.Pat. No. 7,566,476, which is a divisional of U.S. patent applicationSer. No. 10/960,947, filed Oct. 12, 2004, now U.S. Pat. No. 7,125,605,which is a continuation of U.S. patent application Ser. No. 10/642,578,filed on Aug. 19, 2003, now U.S. Pat. No. 6,861,155, which is acontinuation of U.S. patent application Ser. No. 09/625,861, filed onJul. 26, 2000, now U.S. Pat. No. 6,607,829, which claims priority toU.S. Patent Application Ser. No. 60/145,708, filed on Jul. 26, 1999, andis a continuation-in-part of U.S. patent application Ser. No.08/969,302, filed Nov. 13, 1997, now U.S. Pat. No. 6,322,901, each ofwhich is hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.DMR-94-00334 from the National Science Foundation. The government hascertain rights in the invention.

FIELD OF THE INVENTION

This invention relates to luminescent nanocrystalline materials whichemit visible light over a very narrow range of wavelengths. Theinvention further relates to materials which emit visible light over anarrow range tunable over the entire visible spectrum.

BACKGROUND OF THE INVENTION

Semiconductor nanocrystallites (quantum dots) whose radii are smallerthan the bulk exciton Bohr radius constitute a class of materialsintermediate between molecular and bulk forms of matter. Quantumconfinement of both the electron and hole in all three dimensions leadsto an increase in the effective band gap of the material with decreasingcrystallite size. Consequently, both the optical absorption and emissionof quantum dots shift to the blue (higher energies) as the size of thedots gets smaller.

Bawendi and co-workers have described a method of preparing monodispersesemiconductor nanocrystallites by pyrolysis of organometallic reagentsinjected into a hot coordinating solvent (J. Am. Chem. Soc., 115:8706(1993)). This permits temporally discrete nucleation and results in thecontrolled growth of macroscopic quantities of nanocrystallites. Sizeselective precipitation of the crystallites from the growth solutionprovides crystallites with narrow size distributions. The narrow sizedistribution of the quantum dots allows the possibility of lightemission in very narrow spectral widths.

Although semiconductor nanocrystallites prepared as described by Bawendiand co-workers exhibit near monodispersity, and hence, high colorselectivity, the luminescence properties of the crystallites are poor.Such crystallites exhibit low photoluminescent yield, that is, the lightemitted upon irradiation is of low intensity. This is due to energylevels at the surface of the crystallite which lie within theenergetically forbidden gap of the bulk interior. These surface energystates act as traps for electrons and holes which degrade theluminescence properties of the material.

In an effort to improve photoluminescent yield of the quantum dots, thenanocrystallite surface has been passivated by reaction of the surfaceatoms of the quantum dots with organic passivating ligands, so as toeliminate forbidden energy levels. Such passivation produces anatomically abrupt increase in the chemical potential at the interface ofthe semiconductor and passivating layer (See, A. P. Alivisatos, J. Phys.Chem. 100:13226 (1996)). Bawendi et al. (J. Am. Chem. Soc., 115:8706(1993)) describe CdSe nanocrystallites capped with organic moieties suchas tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO)with quantum yields of around 5-10%.

Passivation of quantum dots using inorganic materials also has beenreported. Particles passivated with an inorganic coating are more robustthan organically passivated dots and have greater tolerance toprocessing conditions necessary for their incorporation into devices.Previously reported inorganically passivated quantum dot structuresinclude CdS-capped CdSe and CdSe-capped CdS (Tian et al., J. Phys. Chem.100:8927 (1996)); ZnS grown on CdS (Youn et al., J. Phys. Chem. 92:6320(1988)); ZnS on CdSe and the inverse structure (Kortan et al., J. Am.Chem. Soc. 112:1327 (1990)); and SiO₂ on Si (Wilson et al., Science262:1242 (1993)). These reported quantum dots exhibit very low quantumefficiency and hence are not commercially useful in light emittingapplications.

M. A. Hines and P. Guyot-Sionnest report the preparation of ZnS-cappedCdSe nanocrystallites which exhibited a significant improvement inluminescence yields of up to 50% quantum yield at room temperature (J.Phys. Chem. 100:468 (1996)). However, the quality of the emitted lightremained unacceptable because of the large size distribution of the coreof the resulting capped nanocrystallites.

Danek et al. report the electronic and chemical passivation of CdSenanocrystals with a ZnSe overlayer (Chem. Materials 8:173 (1996)).Although it might be expected that such ZnSe-capped CdSenanocrystallites would exhibit as good as or better quantum yield thanthe ZnS analogue due to the better unit cell matching of ZnSe, in fact,the resulting material showed only disappointing improvements in quantumefficiency (≦0.4% quantum yield).

Thus there remains a need for semiconductor nanocrystallites capable oflight emission with high quantum efficiencies throughout the visiblespectrum, which possess a narrow particle size (and hence with narrowphotoluminescence spectral range).

It is the object of the invention to provide semiconductornanocrystallites which overcome the limitations of the prior art andwhich exhibit high quantum yields with photoluminescence emissions ofhigh spectral purity.

SUMMARY OF THE INVENTION

In one aspect of the invention, a coated nanocrystal capable of lightemission includes a substantially monodisperse core selected from thegroup consisting of CdX, where X═S, Se, Te; and an overcoating of ZnY,where Y═S, Se, and mixtures thereof uniformly deposited thereon, saidcoated core characterized in that when irradiated the particles emitlight in a narrow spectral range of no greater than about 40 nm at fullwidth half max (FWHM). In some embodiments, the narrow spectral range isselected from the spectrum in the range of about 470 nm to about 620 nmand the particle size of the core is selected from the range of about 20Å to about 125 Å.

In other embodiments of the invention, the coated nanocrystal ischaracterized in that the nanocrystal exhibits less than a 10% andpreferably less than 5%, rms deviation in diameter of the core. Thenanocrystal preferably exhibits photoluminescence having quantum yieldsof greater than 30%, and most preferably in the range of about 30 to50%.

In another embodiment of the invention, the overcoating comprises one totwo monolayers of ZnY. The nanocrystal may further comprise an organiclayer on the nanocrystal outer surface. The organic layer may becomprised of moieties selected to provide compatibility with asuspension medium, such as a short-chain polymer terminating in a moietyhaving affinity for a suspending medium, and moieties which demonstratean affinity to the quantum dot surface. The affinity for the nanocrystalsurface promotes coordination of the organic compound to the quantum dotouter surface and the moiety with affinity for the suspension mediumstabilizes the quantum dot suspension.

In another aspect of the invention, a method of preparing a coatednanocrystal capable of light emission includes introducing asubstantially monodisperse first semiconductor nanocrystal and aprecursor capable of thermal conversion into a second semiconductormaterial into a coordinating solvent. The coordinating solvent ismaintained at a temperature sufficient to convert the precursor into thesecond semiconductor material yet insufficient to substantially alterthe monodispersity of the first semiconducting nanocrystal and thesecond semiconductor material has a band gap greater than the firstsemiconducting nanocrystal. An overcoating of the second semiconductormaterial is formed on the first semiconducting nanocrystal.

In one embodiment of the invention, the monodispersity of thenanocrystal is monitored during conversion of the precursor andovercoating of the first semiconductor nanocrystal. In anotherembodiment, an organic overcoating is present on the outer nanocrystalsurface, obtained by exposing the nanocrystal to an organic compoundhaving affinity for the nanocrystal surface, whereby the organiccompound displaces the coordinating solvent.

In addition to having higher quantum efficiencies, ZnS overcoatedparticles are more robust than organically passivated nanocrystallitesand are potentially more useful for optoelectronic devices. The(CdSe)ZnS dots of the invention may be incorporated intoelectroluminescent devices (LEDs). In addition, the (CdSe)ZnS dots ofthe invention may exhibit cathodoluminescence upon excitation with bothhigh and low voltage electrons and may be potentially useful in theproduction of alternating current thin film electroluminescent devices(ACTFELD). In the naming convention used herein to refer to cappednanocrystallites, the compound found within parentheses represents thecore compound (i.e. the bare “dot”), while the compound which followsrepresents the overcoated passivation layer.

These and other features and advantages of the invention are set forthin the description of the invention, which follows.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the absorption spectra of CdSe dots with diametersmeasuring (a) 23 Å, (b) 42 Å, (c) 48 Å and (d) 55 Å before (dashedlines) and after (solid lines) overcoating with 1-2 monolayers of ZnS

FIG. 2 shows the room temperature photoluminescence (PL) spectra of thesamples of FIG. 1 before (dashed lines) and after (solid lines)overcoating with ZnS;

FIG. 3 shows the progression of the absorption spectra for (CdSe)ZnSquantum dots with ZnS coverages of approximately 0, 0.65, 1.3, 2.6 and5.3 monolayers; and

FIG. 4 shows the evolution of the PL for ˜40 Å diameter (CdSe)ZnS dotsof FIG. 3 with varying ZnS coverage.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the preparation of a series of roomtemperature, highly luminescent ZnS-capped CdSe ((CdSe)ZnS)nanocrystallites having a narrow particle size distribution.Nanocrystallites of the present invention exhibit high quantum yieldsgreater than about 30% and preferably in the range of about 30-50% and anarrow band edge luminescence spanning most of the visible spectrum from470 nm to 625 nm. The core of the nanocrystallites is substantiallymonodisperse. By monodisperse, as that term is used herein, it is meanta colloidal system in which the suspended particles have substantiallyidentical size and shape. For the purposes of the present invention,monodisperse particles deviate less than 10% in rms diameter in thecore, and preferably less than 5% in the core.

When capped quantum dots of the invention are illuminated with a primarylight source, a secondary emission of light occurs of a frequency thatcorresponds to the band gap of the semiconductor material used in thequantum dot. As previously discussed, the band gap is a function of thesize of the nanocrystallite. As a result of the narrow size distributionof the capped nanocrystallites of the invention, the illuminated quantumdots emit light of a narrow spectral range resulting in high puritylight. Spectral emissions in a narrow range of no greater than about 60nm, preferably 40 nm and most preferably 30 nm at full width half max(FWHM) are observed.

The present invention also is directed to a method of making cappedquantum dots with a narrow particle size distribution. The cappedquantum dots of the invention may be produced using a two step synthesisin which a size selected nanocrystallite is first synthesized and thenovercoated with a passivation layer of a preselected thickness. Inpreferred embodiments, processing parameters such as reactiontemperature, extent of monodispersity and layer thickness may bemonitored during crystal growth and overcoating to provide a coatedquantum dot of narrow particle size distribution, high spectral purityand high quantum efficiency. “Quantum yield” as that term is usedherein, means the ratio of photons emitted to that absorbed, e.g., thephotoluminescence quantum yield.

The method is described for a (CdSe)ZnS quantum dot, but it isunderstood that the method may be applied in the preparation of avariety of known semiconductor materials. The first step of a two stepprocedure for the synthesis of (CdSe)ZnS quantum dots involves thepreparation of nearly monodisperse CdSe nanocrystallites. The particlesrange in size from about 23 Å to about 55 Å with a particle sizedistribution of about 5-10%. These dots are referred to as “bare” dots.The CdSe dots are obtained using a high temperature colloidal growthprocess, followed by size selective precipitation.

The high temperature colloidal growth process is accomplished by rapidinjection of the appropriate organometallic precursor into a hotcoordinating solvent to produce a temporally discrete homogeneousnucleation. Temporally discrete nucleation is attained by a rapidincrease in the reagent concentration upon injection, resulting in anabrupt supersaturation which is relieved by the formation of nuclei andfollowed by growth on the initially formed nuclei. Slow growth andannealing in the coordinating solvent results in uniform surfacederivatization and regularity in the core structure.

Injection of reagents into the hot reaction solvent results in a shortburst of homogeneous nucleation. The depletion of reagents throughnucleation and the sudden temperature drop associated with theintroduction of room temperature reagents prevents further nucleation.The solution then may be gently heated to reestablish the solutiontemperature. Gentle reheating allows for growth and annealing of thecrystallites. The higher surface free energy of the small crystallitesmakes them less stable with respect to dissolution in the solvent thanlarger crystallites. The net result of this stability gradient is theslow diffusion of material from small particles to the surface of largeparticles (“Ostwald ripening”). Growth of this kind results in a highlymonodisperse colloidal suspension from systems which may initially behighly polydisperse.

Both the average size and the size distribution of the crystallites in asample are dependent on the growth temperature. The growth temperaturenecessary to maintain steady growth increases with increasing averagecrystal size. As the size distribution sharpens, the temperature may beraised to maintain steady growth. As the size distribution sharpens, thetemperature may be raised in 5-10° C. increments to maintain steadygrowth. Conversely, if the size distribution begins to spread, thetemperature may be decreased 5-10° C. to encourage Ostwald ripening anduniform crystal growth. Generally, nanocrystallites 40 Angstroms indiameter can be grown in 2-4 hours in a temperature range of 250-280° C.Larger samples (60 Angstroms or more) can take days to grow and requiretemperatures as high as 320° C. The growth period may be shortenedsignificantly (e.g., to hours) by using a higher temperature or byadding additional precursor materials.

Size distribution during the growth stage of the reaction may beapproximated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. It is also contemplated thatreactants could be added to the nucleation solution during crystalgrowth to grow larger crystals.

The particle size distribution may be further refined by size selectiveprecipitation. In a preferred embodiment, this may be accomplished bymanipulation of solvent composition of the nanocrystallite suspension.

The CdSe nanocrystallites are stabilized in solution by the formation ofa lyophilic coating of alkyl groups on the crystallite outer surface.The alkyl groups are provided by the coordinating solvent used duringthe growth period. The interparticle repulsive force introduced by thelyophilic coating prevents aggregation of the particles in solution. Theeffectiveness of the stabilization is strongly dependent upon theinteraction of the alkyl groups with the solvent. Gradual addition of anon-solvent will lead to the size-dependent flocculation of thenanocrystallites. Non-solvents are those solvents in which the groupswhich may be associated with the crystallite outer surface show no greataffinity. In the present example, where the coordinating group is analkyl group, suitable non-solvents include low molecular weight alcoholssuch as methanol, propanol and butanol. This phenomenon may be used tofurther narrow the particle size distribution of the nanocrystallites bya size-selective precipitation process. Upon sequential addition of anon-solvent, the largest particles are the first to flocculate. Theremoval of a subset of flocculated particles from the initial solutionresults in the narrowing of the particle size distribution in both theprecipitate and the supernatant.

A wealth of potential organometallic precursors and high boiling pointcoordinating solvents exist which may used in the preparation of CdSedots. Organometallic precursors are selected for their stability, easeof preparation and clean decomposition products and low crackingtemperatures. A particularly suitable organometallic precursor for useas a Cd source include alkyl cadmium compounds, such as CdMe₂. Suitableorganometallic precursors for use as a Se source include,bis(trimethylsilyl)selenium ((TMS)₂Se), (tri-n-octylphosphine)selenide(TOPSe) and trialkyl phosphine selenides, such as(tri-n-butylphosphine)selenide (TBPSe). Other suitable precursors mayinclude both cadmium and selenium in the same molecule. Alkyl phosphinesand alkyl phosphine oxide be used as a high boiling coordinatingsolvent; however, other coordinating solvents, such as pyridines,furans, and amines may also be suitable for the nanocrystalliteproduction.

The preparation of monodisperse CdSe quantum dots has been described indetail in Murray et al. (J. Am. Chem. Soc., 115:8706 (1993)), which ishereby incorporated in its entirety by reference.

Next, the CdSe particles are overcoated by introducing a solutioncontaining zinc and sulfur precursors in a coordinating solvent (e.g.,TOP) into a suspension of CdSe nanocrystallites at the desiredtemperature. The temperature at which the dots are overcoated is relatedto the quality of the resultant composite particle. Overcoating the CdSeparticles at relatively higher temperatures may cause the CdSe seedcrystals to begin to grow via Ostwald ripening and deterioration of thesize distribution of the particles leading to broader spectral linewidths. Overcoating the particles at relatively low temperatures couldlead to incomplete decomposition of the precursors or to reducedcrystallinity of the ZnS shell. An ideal growth temperature may bedetermined for each CdSe core size to ensure that the size distributionof the cores remains constant and that shells with a high degree ofcrystallinity are formed. In preferred embodiments, CdSe crystallitesare overcoated using diethyl zinc and hexamethyldisilathiane as the zincand sulfur precursors. CdSe crystallites having a diameter in the rangeof about 23 Å-30 Å are overcoated at a temperature in the range of about135-145° C., and preferably about 140° C. Similarly, nanocrystalliteshaving a diameter of about 35 Å, 40 Å, 48 Å, and 55 Å, respectively, areovercoated at a temperature of about 155-165° C., and preferably about160° C., 175-185° C. and preferably about 180° C., about 195-205° C.,and preferably about 200° C., and about 215-225° C., and preferablyabout 220° C., respectively. The actual temperature ranges may vary,dependent upon the relative stability of the precursors and thecrystallite core and overlayer composition. These temperature ranges mayneed to be modified 10-20° C., depending upon the relative stability ofthe precursors. For example, when the more stable trialkyl phosphinechalcogenides (like TOPSe) are used, higher temperatures are employed.The resulting (CdSe)ZnS composite particles are also passivated withTOPO/TOP on their outermost surface.

The ZnS precursor solution concentration and the rate of its addition tothe CdSe particles is selected to promote heterogeneous growth of ZnSonto the CdSe nuclei instead of homogeneous nucleation to produce ZnSparticles. Conditions favoring heterogeneous growth include dropwiseaddition, e.g., 1-2 drops/second, of the ZnS precursor solution to theCdSe solution and maintenance of the ZnS precursor solution at lowconcentrations. Low concentrations typically range from 0.0005-0.5 M. Insome preferred embodiments, it may be desirable to include a finalpurification step in which the overcoated dots are subjected to sizeselective precipitation to further assure that mainly only (CdSe)ZnScomposite particles are present in the final product.

In other embodiments, it may be desirable to modify the crystalliteouter surface to permit formation of stable suspensions of the cappedquantum dots. The outer surface of the nanocrystal includes an organiclayer derived from the coordinating solvent used during the cappinglayer growth process. The crystallite surface may be modified byrepeated exposure to an excess of a competing coordinating group. Forexample, a dispersion of the capped quantum dot may be treated acoordinating organic compound, such as pyridine, to produce crystalliteswhich dispersed readily in pyridine, methanol, and aromatics but nolonger dispersed in aliphatics. Such a surface exchange process may becarried out using a variety of compounds which are capable ofcoordinating or bonding to the outer surface of the capped quantum dot,such as by way of example, phosphines, thiols, amines and phosphates. Inother embodiments, the capped quantum dots may be exposed to shortchained polymers which exhibit an affinity for the capped surface on oneand which terminate in a moiety having an affinity for the suspension ordispersion medium. Such affinity improves the stability of thesuspension and discourages flocculation of the capped quantum dots.

The synthesis described above produces overcoated quantum dots with arange of core and shell sizes. Significantly, the method of theinvention allows both the size distribution of the nanocrystallites andthe thickness of the overcoating to be independently controlled. FIG. 1shows the absorption spectra of CdSe dots with a particle sizedistribution of (a) 23 Å, (b) 42 Å, (c) 48 Å and (d) 55 Å in diameterbefore (dashed lines) and after (solid lines) overcoating with 1-2monolayers of ZnS. By “monolayer” as that term is used herein, it ismeant a shell of ZnS which measures 3.1 Å (the distance betweenconsecutive planes along the [002] axis in the bulk wurtzite ZnS) alongthe major axis of the prolate shaped dots. The absorption spectrarepresents the wavelength and intensity of absorption of light which isabsorbed by the quantum dot. FIG. 1 indicates a small shift in theabsorption spectra to the red (lower energies) after overcoating due tothe partial leakage of the exciton into the ZnS matrix. This red shiftis more pronounced in smaller dots where the leakage of the exciton intothe ZnS shell has a more dramatic effect on the confinement energies ofthe charge carriers.

FIG. 2 shows the room temperature photoluminescence spectra (PL) of thesamples shown in FIG. 1 before (dashed lines) and after (solid lines)overcoating with ZnS. The PL quantum yield increases from 5-15% for baredots to values ranging from 30% to 50% for dots passivated with ZnS. ThePL spectra are much more intense due to their higher quantum yield of(a) 40%, (b) 50%, (c) 35% and (d) 30%, respectively. The quantum yieldreaches a maximum value with the addition of approximately 1.3monolayers of ZnS. A decrease in quantum yields at higher ZnS coveragesmay be due to the formation of defects in the ZnS shell.

A color photograph demonstrates the wide spectral range of luminescencefrom the (CdSe)ZnS composite quantum dots of the present invention. See,for example, FIG. 3 of U.S. Pat. No. 6,207,229, which is incorporated byreference in its entirety. The photograph shows six different samples ofZnS overcoated CdSe dots dispersed in dilute hexane solutions and placedin identical quartz cuvettes. The samples were irradiated with 356 nmultraviolet light from a uv lamp in order to observe luminescence fromall solutions at once. As the size of the CdSe core increased, the colorof the luminescence shows a continuous progression from the blue throughthe green, yellow, orange to red. Their PL peaks occur at (going fromright to left in FIG. 3 of U.S. Pat. No. 6,207,229) (a) 470 nm, (b) 480nm, (c) 520 nm, (d) 560 nm, (e) 594 nm and (f) 620 nm. In contrast, inthe smallest sizes of bare TOPO-capped dots, the color of the PL isnormally dominated by broad deep trap emissions and appears as faintwhite light.

In order to demonstrate the effect of ZnS passivation on the optical andstructural properties of CdSe dots, a large quantity of ˜40 Å (±10%)diameter CdSe dots were overcoated with varying amounts of Zn and Sprecursors under identical temperatures and variable times. The resultwas a series of samples with similar CdSe cores, but with varying ZnSshell thicknesses. FIG. 3 shows the progression of the absorptionspectrum for these samples with ZnS coverages of approximately 0 (bareTOPO capped CdSe), 0.65, 1.3, 2.6 and 5.3 monolayers. The right handside of the figure shows the long wavelength region of the absorptionspectra showing the lowest energy optical transitions. The spectrademonstrate an increased red-shift with the thicker ZnS overcoating aswell as a broadening of the first peak in the spectra due to increasedpolydispersity of shell thicknesses. The left hand side of the spectrashow the ultra-violet region of the spectra indicating an increasedabsorption at higher energies with increasing ZnS thickness due todirect absorption into the higher ZnS band gap ZnS shell.

The evolution of the PL for the same ˜40 Å diameter CdSe dots with ZnScoverage is displayed in FIG. 4. As the coverage of ZnS on the CdSesurface increases one observes a dramatic increase in the fluorescencequantum yield followed by a steady decline after ˜1.3 monolayers of ZnS.The spectra are red shifted (slightly more than the shift in theabsorption spectra) and show an increased broadening at highercoverages. The inset to FIG. 4 charts the evolution of the quantum yieldfor these dots as a function of the ZnS shell thickness. For thisparticular sample, the quantum yield started at 15% for the bare TOPOcapped CdSe dots and increased with the addition of ZnS approaching amaximum value of 50% at approximately ˜1.3 monolayer coverage. At highercoverages, the quantum yield began to decrease steadily until it reacheda value of about 30% at about 5 monolayers coverage.

Although the invention has been described with reference to thepreparation and performance of CdSe(ZnS), it will be readily apparentthat the method of preparation may be used to obtain monodisperseovercoated quantum dots with various combinations of nanocrystallitecore and overcoating. The method of the invention permits thepreparation of a variety of capped nanocrystals having a very narrowparticle size distribution and exhibiting improvements in color purityand intensity of their photoluminescent emissions. It is contemplatedthat a variety of cadmium chalcogenides, for example, CdX, where X═S,Se, Te may be prepared and overcoated according to the method of theinvention. It is further contemplated that the overcoating may be variedand may include, by way of example only, ZnS, ZnSe, CdS and mixturesthereof.

The invention is described with reference to the following examples,which are presented for the purpose of illustration and which are notintended to be limiting of the invention, the scope of which is setforth in the claims which follow this specification.

Example 1

Preparation of CdSe. Trioctylphosphine oxide (TOPO, 90% pure) andtrioctylphosphine (TOP, 95% pure) were obtained from Strem and Fluka,respectively. Dimethyl cadmium (CdMe₂) and diethyl zinc (ZnEt₂) werepurchased from Alfa and Fluka, respectively, and both materials werefiltered separately through a 0.2 μm filter in an inert atmosphere box.Trioctylphosphine selenide was prepare by dissolving 0.1 mols of Se shotin 100 ml of TOP thus producing a 1M solution of TOPSe.Hexamethyl(disilathiane) (TMS₂S) was used as purchased from Aldrich.HPLC grade n-hexane, methanol, pyridine and n-butanol were purchasedfrom EM Sciences.

The typical preparation of TOP/TOPO capped CdSe nanocrystallitesfollows. TOPO (30 g) was placed in a flask and dried under vacuum (˜1Torr) at 180° C. for 1 hour. The flask was then filled with nitrogen andheated to 350° C. In an inert atmosphere drybox the following injectionsolution was prepared: CdMe₂ (200 microliters, 2.78 mol), 1M TOPSesolution (4.0 mL, 4.0 mmol), and TOP (16 mL). The injection solution wasthoroughly mixed, loaded into a syringe, and removed from the drybox.

The heat was removed from the reaction flask and the reagent mixture wasdelivered into the vigorously stirring TOPO with a single continuousinjection. This produces a deep yellow/orange solution with a sharpabsorption feature at 470-500 nm and a sudden temperature decrease to˜240° C. Heating was restored to the reaction flask and the temperaturewas gradually raised to 260-280° C.

Aliquots of the reaction solution were removed at regular intervals(5-10 min) and absorption spectra taken to monitor the growth of thecrystallites. The best samples were prepared over a period of a fewhours steady growth by modulating the growth temperature in response tochanges in the size distribution, as estimated from the sharpness of thefeatures in the absorption spectra. The temperature was lowered 5-10° C.in response to an increase in the size distribution. Alternatively, thereaction can also be stopped at this point. When growth appears to stop,the temperature is raised 5-10° C. When the desired absorptioncharacteristics were observed, the reaction flask was allowed to cool to˜60° C. and 20 mL of butanol were added to prevent solidification of theTOPO. Addition of a large excess of methanol causes the particles toflocculate. The flocculate was separated from the supernatant liquid bycentrifugation; the resulting powder can be dispersed in a variety oforganic solvents (alkanes, ethers, chloroform, tetrahydrofuran, toluene,etc.) to produce an optically clear solution.

Size-selective Precipitation. Nanocrystallites were dispersed in asolution of ˜10% butanol in hexane. Methanol was then added dropwise tothis stirring solution until opalescence persisted. Separation ofsupernatant and flocculate by centrifugation produced a precipitateenriched with the largest crystallites in the sample. This procedure wasrepeated until no further sharpening of the optical absorption spectrumwas noted. Size-selective precipitation can be carried out in a varietyof solvent/nonsolvent pairs, including pyridine/hexane andchloroform/methanol.

Surface Exchange. Crystallite surface derivatization can be modified byrepeated exposure to an excess of a competing capping group. Heating to˜60° C. a mixture of ˜50 mg of TOPO/TOP capped crystallites and 5-10 mLof pyridine gradually dispersed the crystallites in the solvent.Treatment of the dispersion with excess hexane resulted in theflocculation of the crystallites which are then isolated bycentrifugation. The process of dispersion in pyridine and flocculationwith hexane was repeated a number of times to produce crystallites whichdispersed readily in pyridine, methanol, and aromatics but no longerdispersed in aliphatics.

Example 2

Preparation of CdSe. A second route to the production of CdSe corereplaces the phosphine chalcogenide precursors in Example 1 with(TMS)₂Se. The smallest (˜12 Å) CdSe species are produced under milderconditions with injection and growth carried out at ˜100° C. The productwas further treated as described in Example 1.

Example 3

Preparation of (CdSe)ZnS. Nearly monodisperse CdSe quantum dots rangingfrom 23 Å to 55 Å in diameter were synthesized and purified viasize-selective precipitation as described in Example 1.

A flask containing 5 g of TOPO was heated to 190° C. under vacuum forseveral hours then cooled to 60° C. after which 0.5 mL trioctylphosphine(TOP) was added.

Roughly 0.1-0.4 μmols of CdSe dots dispersed in hexane were transferredinto the reaction vessel via syringe and the solvent was pumped off.

Diethyl zinc (ZnEt₂) and hexamethyldisilathiane ((TMS)₂S) were used asthe Zn and S precursors, respectively. The amounts of Zn and Sprecursors needed to grow a ZnS shell of desired thickness for each CdSesample were determined as follows: First, the average radius of the CdSedots was estimated from TEM or SAXS measurements. Next, the ratio of ZnSto CdSe necessary to form a shell of desired thickness was calculatedbased on the ratio of the shell volume to that of the core assuming aspherical core and shell and taking into account the bulk latticeparameters of CdSe and ZnS. For larger particles the ratio of Zn to Cdnecessary to achieve the same thickness shell is less than for thesmaller dots. The actual amount of ZnS that grows onto the CdSe coreswas generally less than the amount added due to incomplete reaction ofthe precursors and to loss of some material on the walls of the flaskduring the addition.

Equimolar amounts of the precursors were dissolved in 2-4 mL TOP insidean inert atmosphere glove box. The precursor solution was loaded into asyringe and transferred to an addition funnel attached to the reactionflask. The reaction flask containing CdSe dots dispersed in TOPO and TOPwas heated under an atmosphere of N₂. The temperature at which theprecursors were added ranged from 140° C. for 23 Å diameter dots to 220°C. for 55 Å diameter dots. When the desired temperature was reached theZn and S precursors were added dropwise to the vigorously stirringreaction mixture over a period of 5-10 minutes.

After the addition was complete the mixture was cooled to 90° C. andleft stirring for several hours. Butanol (5 mL) was added to the mixtureto prevent the TOPO from solidifying upon cooling to room temperature.The overcoated particles were stored in their growth solution to ensurethat the surface of the dots remained passivated with TOPO. They werelater recovered in powder form by precipitating with methanol andredispersing into a variety of solvents including hexane, chloroform,toluene, THF and pyridine.

In some cases, the as-grown CdSe crystallites were judged to besufficiently monodisperse that no size-selective precipitation wasperformed. Once these CdSe particles had grown to the desired size, thetemperature of the reaction flask was lowered and the Zn and Sprecursors were added dropwise to form the overcapping.

Optical Characterization. UV-Visible absorption spectra were acquired onan HP 8452 diode array spectrophotometer. Dilute solutions of dots inhexane were placed in 1 cm quartz cuvettes and their absorption andcorresponding florescence were measured. The photoluminescence spectrawere taken on a SPEX Fluorolog-2 spectrometer in front face collectionmode. The room temperature quantum yields were determined by comparingthe integrated emission of the dots in solution to the emission of asolution of rhodamine 590 or rhodamine 640 of identical optical densityat the excitation wavelength.

What is claimed is:
 1. A monodisperse population of nanocrystals comprising: a plurality of nanocrystal particles, wherein each particle includes a core including a first semiconductor material and an overcoating including a second semiconductor material deposited on the core; wherein the first semiconductor material and the second semiconductor material are different; wherein the monodisperse population emits light in a spectral range of no greater than about 37 nm full width at half max (FWHM); and wherein the monodisperse population exhibits photoluminescence having a quantum yield of greater than 30%.
 2. The monodisperse population of claim 1, wherein the monodisperse population emits light in a spectral range of no greater than about 30 nm full width at half max (FWHM).
 3. The monodisperse population of claim 1, wherein the cores of the plurality of nanocrystal particles have diameters having no greater than 10% rms deviation.
 4. The monodisperse population of claim 1, wherein the cores of the plurality of nanocrystal particles have diameters having no greater than 5% rms deviation.
 5. The monodisperse population of claim 1, wherein each particle of the monodisperse population further comprises an organic layer on the outer surface of the particle.
 6. The monodisperse population of claim 5, wherein the organic layer includes a moiety selected to provide a stable suspension or dispersion with a suspension or dispersion medium.
 7. The monodisperse population of claim 1, wherein the cores of the plurality of nanocrystal particles have a mean diameter in the range of about 20 Å to about 125 Å.
 8. A monodisperse population of nanocrystals comprising: a plurality of nanocrystal particles, wherein each particle includes a core including a first semiconductor material selected from CdS, CdSe, and CdTe, and an overcoating including a second semiconductor material selected from ZnS, CdS, and CdSe deposited on the core; wherein the first semiconductor material and the second semiconductor material are different; wherein the monodisperse population emits light in a spectral range of no greater than about 37 nm full width at half max (FWHM).
 9. The monodisperse population of claim 8, wherein the monodisperse population emits light in a spectral range of no greater than about 30 nm full width at half max (FWHM).
 10. The monodisperse population of claim 8, wherein the monodisperse population exhibits photoluminescence having a quantum yield of greater than 30%.
 11. The monodisperse population of claim 8, wherein the cores of the plurality of nanocrystal particles have diameters having no greater than 10% rms deviation.
 12. The monodisperse population of claim 8, wherein the cores of the plurality of nanocrystal particles have diameters having no greater than 5% rms deviation.
 13. The monodisperse population of claim 8, wherein each particle of the monodisperse population further comprises an organic layer on the outer surface of the particle.
 14. The monodisperse population of claim 13, wherein the organic layer includes a moiety selected to provide a stable suspension or dispersion with a suspension or dispersion medium.
 15. The monodisperse population of claim 8, wherein the cores of the plurality of nanocrystal particles have a mean diameter in the range of about 20 Å to about 125 Å. 